Vehicle manufacturers are developing hybrid vehicles to meet the demand for more fuel efficient vehicles. One configuration for a hybrid vehicle may be referred to as a Modular Hybrid Transmission (MHT) vehicle design. In a MHT vehicle, an electric machine is sandwiched between a conventional automatic step ratio transmission and the engine. The electric machine is attached to the transmission impeller or input shaft. The engine is selectively disconnected from the transmission using a disconnect clutch. The disconnect clutch allows the vehicle to be driven under electric power alone, in hybrid mode with both the electric machine and the engine propelling the vehicle, or in a combustion engine only mode in which the vehicle is propelled by the engine only.
Regenerative braking provides an approach to increasing the range and fuel economy in MHT vehicles. Kinetic energy of the vehicle is converted to electricity used to charge a high voltage battery using an e-machine as a brake and a generator. Since regenerative brake efficiency drops off at lower speeds, a multi-speed geared automatic transmission of the MHT system is downshifted to increase the e-machine speed and increase the available torque and efficiency. The time required to downshift the MHT system reduces the time available to generate electricity by regenerative braking because maintaining acceptable shift quality requires a significant reduction of regenerative braking.
This disclosure is directed to the problem of improving regenerative braking efficiency and other problems as summarized below.